Here's an image I took with the full frame Nikon D700, and a Sigma 10-20mm lens meant for a crop sensor. The strong vignetting is a result of the mismatch. For those unfamiliar with photography jargon, this lens is designed to let in a more narrow beam of light, for smaller sensors. And I used a camera body with a wider sensor than that beam of light. The result is the black corners, where no light hit the sensor.
I think the vignetting works for this image. The composition is a single-point perspective that draws the eye to the center, and the vignetting reinforces that, giving the image a lost-in-space feel. What do you think? Would this image be improved by cropping?